netdev - Re: [NET] ROUTE: fix rcu_dereference() uses in /proc/net/rt

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Date:	Fri, 11 Jan 2008 00:10:42 +0100
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	davem@...emloft.net, dipankar@...ibm.com, netdev@...r.kernel.org
Subject: Re: [NET] ROUTE: fix rcu_dereference() uses in /proc/net/rt_cache

Eric Dumazet wrote, On 01/09/2008 11:37 AM:
...
> [NET] ROUTE: fix rcu_dereference() uses in /proc/net/rt_cache
...
> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> index d337706..28484f3 100644
> --- a/net/ipv4/route.c
> +++ b/net/ipv4/route.c
> @@ -283,12 +283,12 @@ static struct rtable *rt_cache_get_first(struct seq_file *seq)
>  			break;
>  		rcu_read_unlock_bh();
>  	}
> -	return r;
> +	return rcu_dereference(r);
>  }
>  
>  static struct rtable *rt_cache_get_next(struct seq_file *seq, struct rtable *r)
>  {
> -	struct rt_cache_iter_state *st = rcu_dereference(seq->private);
> +	struct rt_cache_iter_state *st = seq->private;
>  
>  	r = r->u.dst.rt_next;
>  	while (!r) {
> @@ -298,7 +298,7 @@ static struct rtable *rt_cache_get_next(struct seq_file *seq, struct rtable *r)
>  		rcu_read_lock_bh();
>  		r = rt_hash_table[st->bucket].chain;
>  	}
> -	return r;
> +	return rcu_dereference(r);
>  }

It seems this optimization could've a side effect: if during such a
loop updates are done, and r is seen !NULL during while() check, but
NULL after rcu_dereference(), the listing/counting could stop too
soon. So, IMHO, probably the first version of this patch is more
reliable. (Or alternatively additional check is needed before return.)

Regards,
Jarek P.
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